In cigarette packing, stamps are fed successively to a pickup member of a packing machine by a feed device, in which the stamps are stacked inside a substantially horizontal channel having an outlet bounded laterally by shoulders and facing the pickup member.
In feed devices of this sort, a given thrust is exerted on the stack to compress it against the shoulders of the outlet and so ensure correct engagement of the stack by the pickup member.
Tests show that withdrawal of the stamps one by one, as opposed to groups of two or more at a time, is substantially only ensured when the stamps at the end of the stack facing the outlet are subjected to substantially constant pressure over and above a given value, and that this constant pressure cannot be achieved by simply exerting constant thrust on the stack in the direction of the outlet. The stack, in fact, when compressed, is deformed elastically, with an elastic response which, for a given material and thickness of the stamps, depends on the length of the stack, so that the pressure with which the stack adheres to the shoulders of the outlet varies as the stack is used up, even if the stack is subjected to constant thrust.
It has been proposed to eliminate this drawback by continuously determining the contact pressure of the stack on the lateral shoulders of the outlet, and applying thrust by means of a variable-thrust pressure member feedback-controlled to keep the contact pressure equal to a given reference value.
Unfortunately, this solution, too, has not been altogether successful. The stack, in fact, when compressed, behaves like an elastic block with a relatively high degree of hysteresis varying with the length of the stack, and feedback thrust control causes the contact pressure to assume a mean value substantially equal to the reference value, but to oscillate continually and uncontrollably about the reference value, thus preventing withdrawal of the stamps one by one.